A circle is a simple closed shape. It is the set of all points in a plane that are at a given distance from a given point, the centre; equivalently it is the curve traced out by a point that moves so that its distance from a given point is constant.

Radius: R

Diameter: d

Chord: a

Secant segment: e,f

Tangent segment: g

Central angle: α

Inscribed angle: β

Perimeter: L

Area: S

- $a=2R\ sin\ α/2$

- $a_1a_2=b_1b_2$

- $ee_1=ff_1$

- $g^2=ff_1$

- $β=α/2$

- $L=2πR=πd$
- $S=πR^2={πd^2}/4={LR}/2$

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